The present invention relates to multistage flash evaporators and more particularly to distillate collection in flash evaporators of the cross-flow and paired-stage types.
In a conventional multistage flash (MSF) evaporator, heated brine is passed through a series of stages, from the highest to the lowest temperature stage, with a portion of the brine flashing into vapor in each stage. The vapor formed is then passed through a mesh separator and condensed on tubes in the same stage. In a paired-stage or cross-flow type MSF evaporator, the distillate formed in each stage is normally collected in a distillate tray and drained into a duct located at one end of the condenser tube bundle. The duct passes under each stage's distillate tray, cascading stage-to-stage until the total accumulated flow is removed from the lowest temperature stage by the distillate pump.
This invention provides a simple method for isolating and rejecting unacceptable distillate produced in individual stages of an MSF evaporator. This allows production to continue (at some reduction in output) until an outage can be scheduled to correct the problem. Without this invention, the alternative is to immediately shut down the unit whenever a purity problem occurs, with loss of all distillate production until the problem can be corrected. In many instances, this alternative is completely unacceptable.
In operation of multistage flash evaporators, the development of unacceptable contaminated distillate from one or more stages typically requires shutdown of the entire unit until the impurity problem is corrected. Contamination can result for example from a tube leak or from brine carryover caused by a displaced mesh. Shutdown of the entire distillation process for a localized impurity problem may be virtually unacceptable because of existing demand for water production, or at the very least it interferes with economic plant maintenance management based on a scheduling of plant shutdowns.
It has thus been desirable to develop a reliable and preferably simple scheme for blocking the collection of distillate flow from any individual stage in which the flow is determined to be impure (i.e., contains brine). In this manner, repair of the impure stage(s) can be economically deferred until a plant maintenance shutdown is scheduled.
Some effort has been applied in the prior art to address the stage distillate impurity problem. For example, in U.S. Pat. No. 3,713,989 by P. R. Bom, a scheme is shown for isolating stage distillate flows. The scheme is structurally involved, requires a relatively large space under each tube bundle and is directed to long flow plants.